Process of prescreening photographic material involving rehalogenation



I 1957 R E. MAURER 2,805,157

PROCESS OF PRESCREENING PHOTOGRAPIflC MATERIAL. INVOLVING REHALOGENATION Filed July 1, 1955 2 Sheets-Sheet 2 2 EXPQSURE ISTEXPOSURE Ricbarz lEMazlrer INVENTOR.

Milk

Unite States Patent PROCESS OF PRESCREENING PHOTOGRAPHIC MATERIAL INVOLVING REHALOGENATION Richard E. Maurer, Rochester, N. Y., assignor to Eastman Kodak Company, Rochester, N. Y., a corporation of New Jersey Application July 1, 1955, Serial No. 519,355

4 Claims. (Cl. 9645) This invention relates to prescreened photographic film or paper; that is, photographic material which can be processed directly to a halftone image after exposure to a continuous tone image. Prescreened photographic materials of the variable sensitivity type are described in U. S. 2,691,583 Maurer and U. S. 2,691,586 Yule et al.

The object of the present invention is to provide a prescreened material which has an exceptionally long scale or range and which has good keeping qualities or so-called good shelf life qualities. Also, these prescreened materials are practically free of so-called reciprocity efiects.

Another object of the invention is' to provide a prescreening process which works with all types ofsilver halide emulsions.

According to the present invention, a prescreened material is made up which actually has two different silver halides distributed throughout the material to provide the ditferent degrees of sensitivity required for this type of prescreening. Ordinary silver halide as used in photographic film or paper has a high sensitivity produced by known techniques in preparation, which may include proper treatment of the gelatin or othe r vehicle, proper aging and possibly optical sensitizing or hypersensitizing. The present invention works with all types of emulsions, but certain types are preferred, because of their other properties. For example, one may desire a high contrast prescreened material, in which case, one selects a high contrast emulsion with a gamma greater than 2 for prescreening. As another example, one may desire an orthochromatic or panchromatic prescreened material, in which case one selects an orthochromatic or panchromatic emulsion. As still another example one may desire a prescreened projection paper, in which case, one selects a projection paper emulsion. The type of emulsion to be used does not impose any limitations on the present invention although the reproduction scale or range and final emulsion speed depend on the particular emulsion selected. I

If one exposes and develops a silver halide emulsion to produce grains of metallic silver, one may rehalogenize the silver to form silver halide again. This rehalogenized silver is, in general, much less sensitive than the original silver halide.

According to the present invention, a prescreened photographic material is made up of a support layer of film or paper with a uniformly thick layer ofsilver halide emulsion thereon, consisting of rehalogenized silver halide distributed throughout the rest of the silver,

halide in accordance with a vignetted halftone pattern.

When this material is exposed'to a continuous tone image, the brightest parts of the image produce a latent image in all of the regular silver halide and also all of the rehalogenized or regenerated silver halide. The middle tones in the image produce the latent image in only part of the two emulsions (mainly in the original silver halide) and'the darkest points of the image produce a useful or 2,805,157 Patented Sept. 3, 1957 "ice 2. specified density only in the middle of each dot which consists solely of the original silver halide.

In general, optical sensitization of the original emulsion (e. -g., orthochromatic or panchromatic sensitization) carries over to the rehalogenized emulsion.

In general, prescreened material should have a high contrast for halftone work in order to insure sharp dot structure.

The method of rehalogenation to produce this type of prescreened material consists merely of exposing an emulsion layer on film or paper through a halftone screen which may be either a ruled screen out of contact with the emulsion or a so-called contact screen in contact with the emulsion. Contact screens have vignetted dot densities. The emulsion layer is then developed to produce metallic silver grains distributed in accordance with the halftone pattern among the unexposed silver halide grains in the emulsion. The developer may be any one normally used for the emulsion in question or may be a different one if special eife'cts are desired. There is nothing peculiar about this step- It is merely standard development as used in ordinary photographic processes. For example, in' the preferred embodiments of the invention one chooses a developer which gives a useful speed and average contrast to the emulsion, just as one ordinarily would. Extremely high or extremely low contrast gives' less favorable tonal qualities to the prescreening'. If two developers are otherwise more or less equivalent and one of them produces a silver image which is more easily rehalogenized than the image produced by the other, the former is preferred since it permits the use of a weaker bleach bath which in turn tends to cause less reduction in emulsion speed at the centers of the dots (where no rehalogenization is present).

After this normal development, the emulsion layer is washed to remove the developer, but is not fixed. It is then bleached in a bath containing a halide ion (for example, a bath containing potassium bromide) which bath rehalogenizes the metallic silver to form silver halide of lower sensitivity than the unexposed silver halide. The film is then dried and constitutes prescreened material. This method of prescreening is particularly suitable for continuous prescreening of roll film or roll paper. No short exposures such as those required for the Clayden effect are involved. Hence the screening exposure can be applied through a suitable roller and then the film can be passed through a series of baths.

It is realized that the explanation of the invention given above suggests that the film consists of two silver halides, each of one sensitivity, but when one realizes that other factors such as grain size provide a range of individual sensitivities for each set of grains, the continuously varying sensitivity across the dots is readily understood. In any case, that is exactly the effect one gets in practice and 'it is the effect desired. The exact mechanism which causes the difierence in sensitivity between the original silver halide and the rehalogenized silver halide is, of course, not fully understood. In fact, it is believed that certain grains may be partly original and partly rehalogenized and have a lower sensitivity than that of the original grain. Rehalogenized would include such partly rehalogenized grains if present. Also, the silver halides are different even if, as pointed out above, the mechanism of the difference is not known.

The invention will be more fully understood from the following description when read in connection with the accompanying. drawings,- in which:

Figs. 1A through 1E constitute a flow chart showing the steps in the manufacture of prescreened film according to the invention; 7 r

Fig. 2 illustrates an alternative. for the exposure step of the process shown inFig. 1A;

Fig. 3 represents schematically the application of the invention to continuous prescreening; and

Fig. 4 illustrates the principle of the invention by means of a crossed step tablet sensitometer strip.

In Fig.1 light from a lamp it) through suitable aperture 11 and a ruled halftone screen 12 exposes a photographic emulsion 15 carried on a film support 16. The halftone pattern or light striking the emulsion 15 is vignetted since the sharp screen 12 is out of contact with the emulsion 15. The film is then placed in a developer 17 which may be any developer selected in accordance With the film used or in accordance with any other properties desired, not related to the present invention. The silver halide in the emulsion 15 is partly changed to metallic silver in the layer now labeled as in Fig. 1B. The distribution of density is represented by a wavy line l8, but, of course, the grains are not necessarily dis tributed in this way in the layer 25. There are more silver grains developed in the areas which were behind the openings in the screen 12 than there are in the areas behind the opaque areas in the screen 12. The distribution or" silver grains in the layer 25 is in accordance with a vignetted halftone pattern.

The emulsion 25 is then washed in a water bath 20 to remove the developer. It is then placed in a bleach bath 21 containing a silver halide ion. For example, the bleach bath may have the following composition:

Potassium bromide gms 50 Potassium ferricyanide grns 50 Potassium ferrocyanide gms 12.5 Water to make ccs 1000 The present invention is not limited to this particular bleach bath or to any particular bleach bath, but the bath, of course, must be one which rehalogenizes the metallic silver grains.

In this connection it is noted that the exposures necessary to produce the latent halftone image which is developed to silver grains need not be abnormally high. It is merely the exposure normally required for exposing the particular film being used or even less. This, of course, distinguishes directly from the Albert reversal system described in U. S. Patent 2,691,583 Maurer mentioned above.

The present invention is not restricted to any particular type of rehalogenation bath. Such baths normally con tain an oxidizing agent (in the above example, potassium lerricyanide) and a halide.

The rehalogenation or bleaching bath 21 converts the emulsion layer to that shown at 26 containing at least two different silver halides, one only of which is rehalogenized silver halide. The film is then dried to form a presereened material having emulsion layer 27 whose sensitivity as illustrated by the broken line 23 varies across the film in accordance with a vignetted halftone pattern. Since the prescreening is made up of silver halides of differing sensitivities rather than some form of latent image which might fade or grow on aging, the stability of the prescreened material is high. The drawings illustrate a film base 16 as the support, but the support may be of paper for making halftone prints. It is noted that the range or scale of sensitivity of the prescreened material depends on the sensitivity of the rehalogenated emulsion and even if this rehalogenized emulsion had zero sensitivity, the material would still be a prescreened one suitable for some purposes. The greater the difference in sensitivity between the two emulsions, the greater the density scale which the material can reproduce.

Fig. 2 merely illustrates the fact that a contact screen 31 may be used in place of the ruled screen 12 for exposing theemulsion layer 15. The light rays are indicated by arrows 32. The beam of light need not be collimated or focused in any way since a contact print is being made in this case.

In Fig. 3 a roll of'filrn or paper 35 unwinds from a 2,805,157 M W g roll 36 with the emulsion surface facing outward. It passes over a roller 37 and under a drum 38, the surface of which is transparent and includes a vignetted halftone pattern similar to that in a contact screen. Light from a lamp 3? exposes the film 35 through the halttone screen 33. That is, the rotating drum 38 presents an illuminated area containing a vignetted halftone screen pattern of light. This area is elongated, being equal to the length of the drum 33. As the drum rotates, this pattern of light moves constantly across the area and the emulsion layer moves through the area synchronously with the light pattern since it is rolling in contact with the drum at all times as it is being exposed. Light from the lamp 39 is confined by a shield 34 to illuminate only the bottom of the drum 33 as it rotates.

The film then passes over rollers 40 and into :1 development bath 41 which produces a silver image in the exposed areas. The film then passes over rollers 42 into a wash bath 43 which removes the developer from the film, but does not fix the film. At this stage the film contains silver grains distributed among silver halide grains in the emulsion. The film then passes over rollers 44 and into a rehalogenizing bleach bath 45 of the type described above. At this stage the silver grains are replaced by silver halide but this particular silver halide has much lower sensitivity than the original silver halide grains. The film then passes over roll 46 and is dried in loops 47 before being wound onto a take-up roll 48.

Fig. 4 illustrates'a crossed step tablet sensitometer test. The crossed step tablet principle is now well known. In a simple form, it consists merely of giving a first exposure through a step tablet and then giving a second exposure through the same step tablet rotated through The interaction of the two exposures for all intensities of each can thus easily be studied. There may be processing steps between the two exposures. In the present case there is only one exposure involved in the prescreening. The second exposure merely represents the continuous tone exposure made by the customer who uses the prescreened material. The film 51 has received a first exposure in which the intensity increases in steps toward the right, thus the righthand column has received the maximum first exposure and the lefthand column can be considered as having received no first exposure. The film is then developed, washed and bleached to reconvert any silver thus produced to rehalogenized silver halide. Thus if the film were fixed at this stage without any second exposure, there would theoretically be no density many of the steps. Actually rehalogenation might not be complete so that the righthand column or maybe the two columns at the extreme right might show a faint fog density. However, this is of no interest in the present invention and can be avoided either by insuring complete rehalogenation or by avoiding the areas represented by the two righthand columns. That is, the first'exposure need never be extremely high. The lefthand columns 53 may be cons'idered as having received 'no first exposure and hence in these areas the silver halide remains in its original state, except possibly for some minor efiects due to having been immersed in the various baths.

The second exposure increases from the bottom toward the top of the strip 51. As shown in columns 53, the material in the areas which have received no first exposure has a certain standard sensitometric response. However, as shown in the area 52, which is the area in which the present invention operates, the response of the film to the second exposure falls off as one moves to the right. That is, the sensitivity of the material to the second exposure is much less in those areas which have received a large first exposure and in which the silver halide is increasingly made up of rehalogenized silver halide.

In any particular halftone dot area, the center of the dot corresponds to columns 53 and the corners of the dot correspond to some particular column further to the right, depending on the scale or range of sensitivities desired.

The range can be pretty well selected at will and is not limited or restricted as in prescreened films which depend on some particular photographic effect such as the Clayden effect, the Albert efiect, etc. A relatively high intensity is required to expose the corners of such prescreened film whereas a much lower intensity is required for the middle tones or the centers of the dots. Also, the tonal reproduction can be controlled more or less at will by proper selection of the distribution in density in the original exposing screen 31, for example, in Fig. 2.

I claim:

l. The method of prescreening a silver halide emulsion layer which comprises exposing the layer through a halftone screen to substantially uniformly distributed light, developing the layer to metallic silver grains distributed in accordance with a halftone pattern among the unexposed silver halide grains in the emulsion, washing the layer, bleaching the layer in a bath containing halide ion to rehalogenize the metallic silver to silver halide of lower sensitivity than the unexposed silver halide and drying the layer.

2. The method of continuously prescreening a roll of photographic material having a silver halide emulsion layer, which comprises illuminating an elongated area with a substantially uniformly distributed vignetted halftone screen pattern of light moving constantly across the area, passing the emulsion layer from the roll through said area synchronously with the light pattern to expose the layer, then passing the layer through a developer bath to form in the layer metallic silver grains distributed in accordance with a halftone pattern among the unexposed silver halide grains in the emulsion, then passing the layer through a wash hath, then passing the layer through a bleach bath containing halide ion to rehalogenize the metallic silver to silver halide of lower sensitivity than the unexposed silver halide and then drying the layer.

3. A prescreened photographic material comprising a support layer and a silver halide emulsion layer prescreened by the method of claim 1.

4; A prescreened photographic material made according to the method of claim 2.

References Cited in the file of this patent UNITED STATES PATENTS 2,691,583 Maurer Oct. 12, 1954 

1. THE METHOD OF PRESCREENING A SILVER HALIDE EMULSION LAYER WHICH COMPRISES EXPOSING THE LAYER THROUGH A HALFTONE SCREEN TO SUBSTANTIALLY UNIFORMLY DISTRIBUTED LIGHT, DEVELOPING THE LAYER TO METALLIC SILVER GRAINS DISTRIBUTED IN ACCORDANCE WITH A HALFTONE PATTERN AMONG THE UNEXPOSED SILVER HALIDE GRAINS IN THE EMULSION, WASHING THE LAYER, BLEACHING THE LAYER IN A BATH CONTAINING HALIDE ION TO REHALOGENIZE THE METALLIC SILVER TO SILVER HALIDE OF LOWER SENSITIVITY THAN THE UNEXPOSED SILVER HALIDE AND DRYING THE LAYER. 